1. Field of the Art
The present invention relates to a catalyst for the polymerization of olefins. More particularly, the present invention relates to the polymerization of olefins, in which by using a specific catalyst for the polymerization of olefins, especially u-olefins having at least 3 carbon atoms, highly stereoregular polymers can be prepared under stable polymerization conditions industrially advantageously.
Furthermore, the present invention relates to a process in which propylene or a mixture of propylene and ethylene is polymerized in the presence of a specific catalyst through at least two steps to prepare a propylene copolymer, especially a propylene block copolymer.
According to the process of the present invention, a propylene copolymer having high rigidity, high impact strength and good flowability can be prepared with a high activity while controlling formation of a low crystallinity by-product.
Moreover, the present invention relates to a process in which an olefin is polymerized in the presence of a specific catalyst, and a solvent used in the polymerization and left after separation and recovery of the polymer from the reaction mixture is recycled to the polymerization step and used again for the polymerization, whereby a polyolefin can be prepared at a high efficiency.
A conventional catalyst for the polymerization of olefins, which comprises a solid catalyst component comprising titanium, magnesium and a halogen as the indispensable ingredients and an organoaluminum compound has a very high activity but is defective in that if the stereoregularity of the polymer is important, an electron donor compound should be used in the polymerization process.
However, when a catalyst comprising an electron donor compound as the third component, viz. an external electron donor, is used, the polymerization velocity is reduced, since the electron donor compound reacts with the organoaluminum compound, and if the polymerization temperature is elevated in an effort to increase the productivity of the polymer, this undesired reaction is also promoted and, therefore, increase of the polymerization quantity, viz. increase of the production efficiency, is limited. Accordingly, the catalyst is defective in that it is difficult to control the molecular weight and other properties of the polymer produced thereby.
2. Prior Art
Under this background, development of a catalyst system not comprising an electron donor compound as the third component, an external electron donor, capable of providing a highly stereoregular polymer in a high catalyst yield, is desired for solving the foregoing problems.
For example, Japanese Patent Application Laid-Open Specification No. 138715/1983, equivalent to U.S. Pat. No. 4,442,276, discloses a process in which the polymerization is carried out in the presence of a catalyst system which is formed of an organoaluminum compound and a solid component obtained by reacting (1) a titanium composite comprising tetravalent titanium, magnesium, a halogen and an electron donor as the indispensable ingredients with (2) an organic silicon compound having an Si--O--C linkage in the presence of an organoaluminum compound or by treating the above-mentioned titanium composite with an organoaluminum compound and reacting the thus treated titanium composite with the organic silicon compound, without using an external electron donor.
The above-mentioned problems may be solved by this proposal, but, to the best of our knowledge, the performance of the product polymer is limited, the catalyst is degraded with the lapse of time and the ratio of the amount of the titanium component to the organoaluminum compound should be controlled at the polymerization. Therefore, further improvements are required.
As means for improving the impact strength, especially the impact strength at low temperatures, in crystalline polypropylene which is excellent in rigidity, heat resistance and the like, there is known a process in which propylene is copolymerized stepwise with ethylene or another olefin to form a block copolymer by, e.g. Japanese Patent Publictions No. 11,230/1968, No. 16,668/1969, No. 20,621/1969, No. 24,593/1974 and No. 30,264/1974 and Japanese Patent Application Laid-Open Specification No. 25,781/1973, No. 115,296/1975, No. 35,789/1978 and No. 110,072/1979.
When propylene is copolymerized with ethylene in at least two stages, the impact resistance is improved, but since the product contains a copolymerization component, the process involves an industrial problem in that a low crystallinity polymer of little value is formed as a by-product in a large quantity. Accordingly, various trials have been made to reduce the amount of the low crystallinity polymer formed as the by-product.
A catalyst based on titanium trichloride is well-known in the art as the catalyst for the stereoregular polymerization of olefins. However, the activity of this catalyst is not very high, and the catalyst-removing step may thus be necessary on the polymer produced.
The technique of introducing a magnesium compound into the solid catalyst component for enhancing the activity to such an extent that the catalyst-removing step will be unnecessary is known. Furthermore, the technique of using an electron donor as an ingredient of the solid catalyst component or as a polymerization additive for controlling formation of a low crystallinity component as a by-product is known by Japanese Patent Application Laid-Open Specifications No. 9,842/1972, No. 126,590/1975 and No. 57,789/1976.
Processes for preparing propylene block copolymers by using these catalyst systems have been proposed by, e.g. Japanese Patent Application Laid-Open Specifications No. 98,045/1977 and No. 88,049/1978. However, these processes may, to the best of our knowledge, be still insufficient from the practical viewpoint because a low crystallinity component may be formed in a large amount as a by-procut.
As means for overcoming this defect, there has been proposed a process in which an organic silicon compound having an Si--O--C or Si--O--N linkage is used as a polymerization additive by Japanese Patent Application Laid-Open Specification No. 83,016/1983, equivalent to U.S. Pat. No. 4,547,552. However, since the molecular weight of the rubbery portion formed which is an ethylene/propylene copolymer or the polyethylene portion is not very high, this process may involve the following problems:
(1) The formed rubber is easily extracted from the polymer and acts as a low crystallinity component formed as a by-product, and the formed polymer can get sticky and cohesion or sticking of the polymer can readily be caused. Therefore, an operation trouble often takes place.
(2) The impact resistance at low temperatures is not improved.
(3) In order to maintain MFR of the whole polymer at a high level, the molecular weight of the crystalline polypropylene portion should be inevitably increased. However, this increase results in degradation of the spiral flow, that is, the flowability of the polymer in a mold.
In the case where a polyolefin is prepared by the liquid phase polymerization, on the other hand, it is conventional to recycle a polymerization solvent used to the polymerization step after it has undergone the after-polymerization steps such as the catalyst removing step and a purification step such as distillation for removing polymerization by-products and water or an alcohol incorporated at the catalyst-removing step.
Since the manufacturing cost of a polyolefin is increased by this solvent purification, omission of the solvent purification step is desired. For this purpose, it is indispensable that the amounts of polymerization by-products should be sufficiently small and the step of removing water or an alcohol which acts as a poison to the catalyst should become unnecessary.
In fact, in the polymerization in which a catalyst based on titanium trichloride catalyst is used, recycle of a polymerization solvent while omitting the solvent purification step has been tried as shown in Japanese Patent Publication No. 96/1961 and Japanese Patent Application Laid-Open Specifications No. 83,784/1974 and No. 79,589/1975. So far as we know, however, in this process, the polymerization activity is reduced or increase of formation of by-products is caused as the polymerization solvent is utilized repeatedly, and the process is not industrially sufficient.
Various catalyst systems comprising a magnesium compound as a carrier for attaining such a high activity as rendering the catalyst removal unnecessary or comprising an electron donor as a third polymerization component for improving the stereoregularity and reducing formation of polymerization by-products have been reported. Moreover, there have been proposed processes in which these catalyst systems are used and the polymerization solvent is recycled to the polymerization step without the solvent purification step by Japanese Patent Application Laid-Open Specifications No. 31,910/1982 and No. 138,707/1983.
In these processes, to the best of our knowledge, the reaction of the electron donor as the third polymerization component with the organoaluminum compound component cannot be neglected and the polymerization stability is degraded, and if the recycle is repeated for a long time, the quality of the polymer becomes different from the quality of the initially formed polymer. Moreover, the polymerization activity is reduced by influences of the reaction product. Furthermore, the reaction product is accumulated and the smell of the polymer is worsened. Still further, the polymerization by-product is accumulated in the polymerization solvent while it is recycled, and the polymerization by-product is brought into the product polymer to reduce the properties of the product, such as the anti-blocking property and the rigidity, for example, in case of a film.
Japanese Patent Application Laid-Open Specification No. 138,715/1983, equivalent to U.S. Pat. No. 4,442,276 discloses a polymerization process in which an electron donor as the third polymerization component is not used. However, this process may be defective so far as we know in that the polymerization stability is poor and when the polymerization is conducted for a long time, the activity is reduced and formation of the polymerization by-product is increased. Furthermore, when the polymerization solvent is recycled and used repeatedly, the catalyst activity is similarly reduced.